1. Field of the Invention
The present invention relates to gradation reproduction, used in for example, a digital copying machine or a printer, for reproducing a gradation image by using a matrix pattern which changes a dot area ratio according to gradation level.
2. Description of Related Art
Gradation reproduction methods have been proposed for recording a half-tone image for an apparatus such as a is printer or a digital copying machine by using various area gradation such as density pattern method or dither method. In the density pattern method, an image data of a pixel in an original image is compared with a threshold value at each dot in an m*n matrix pattern (density pattern), and gradation image is reproduced by outputting a pattern image having a dot area ratio in correspondence to the gradation of the pixel. In the dither method, an image data of each pixel in an original image is compared with a threshold value at a corresponding dot in an m*n matrix pattern (threshold matrix), and gradation image is reproduced by outputting a pattern image having a dot area ratio in correspondence to the gradation of each pixel. Further, a gradation reproduction method (IH method) is proposed by combining a matrix modified based on a threshold matrix of fattening type with pulse width modulation of laser beam exposure in an electrophotographic process. The number of gradation levels is as large as m*m*L*0.5+1 including white where L represents dot division number in pulse width modulation and m*m represents matrix size. Therefore, an image of higher quality than the previous dither method can be obtained.
In these gradation reproduction methods, gradation is reproduced by controlling the total exposure time by a is light source or the number of dots for light emission in a unit area. On the other hand, a generalized dither method is proposed for gradation reproduction by controlling the exposure position (Japanese Patent laid open Publication 7-115538/1995). This uses a fact that even if the total exposure time is the same, the accumulated energy is different so that the image area becomes different when the exposure time is different.
In any of the gradation reproduction methods, a matrix pattern for changing dot area ratio according to the gradation is stored in a storage device, and the pattern is read from the storage device by pointing the X and Y addresses.
However, it is a problem that the storage capacity of the storage device for storing the pattern is large for the gradation reproduction methods because this increases the cost thereof. As one pattern is needed for each gradation level, 256 patterns are provided for 256 gradation levels. Further, gradation reproduction of a color image, different patterns are needed for the four color components of cyan, magenta, yellow and black, or the memory capacity becomes four times that of one color. Further, because higher image quality is desired recently, patterns exclusive for each image type such as character image or photograph image are needed. In this case, the storage capacity is increased further.